Blender 2.79 smoke simulation, domain rendering as solid cube
The Quick Smoke option automatically generates the smoke material based on your currently selected Render Engine. The Blender Render renderer uses different materials to Cycles and they are not interchangeable. Therefore, if you do 'Quick Smoke' whilst Blender Render is selected and then swap to Cycles, you will not have a valid material (since it will be the Blender Render material, rather than Cycles) and you will see the Cycles material rendered as the default (Diffuse) material.

The solution is to select Cycles renderer before you select Quick Smoke. The easiest way to resolve this will be to delete your Smoke Domain, select your Cube again, and then re-select Quick Smoke - this should create a new domain with the correct smoke material.